High frequency dielectric heating process and apparatus



C. GROS July H, 1967 HIGH FREQUENCY DIELECTRIC HEATING PROCESS ANDAPPARATUS Filed Nov. 30, 1964 United States Patent 3,330,932 HIGHFREQUENCY DIELECTRIC HEATING PROCESS AND APPARATUS Chajim Gros, 39Alexandra Road, London, England Filed Nov. 30, 1964, Ser. No. 414,602Claims priority, application Great Britain, Nov. 30, 1963, 47,361/ 63 8Claims. (Cl. 219-10.61)

ABSTRACT OF THE DISCLOSURE A high frequency heating process forsupplying power across the whole width of a dielectric material duringlongitudinal movement of the material between electrodes in which one ofthe electrodes is divided with respect to the high frequency power intoa plurality of parts in such a way that each part covers a portion ofthe material not covered by another part during its passage between theelectrodes.

This invention relates to a high frequency dielectric heating processand apparatus.

One of the problems encountered in the operation of such processes isthat it is normally necessary when an increase is made in the area ofdielectric material to be heated to increase the area of the electrodesbetween which the dielectric material is placed and the power suppliedto the electrodes to avoid amongst other things the generation ofstanding waves and to decrease the frequency of the source of the power.This results in an increase in the voltage stress across the dielectricmaterial to achieve satisfactory heating and the consequent increase inthe possibility of an electrical breakdown of the insulation between theelectrodes.

The present invention provides a process and apparatus in which thepossibility of an electrical breakdown in the insulation between theelectrodes occurring is reduced and in which use may be made of a higherfrequency source than would otherwise be possible. The term highfrequency where used in this specification and claims is not limited, inits application to any one band of [frequencies but it may include forexample frequencies in the very high and ultra high frequency bands.

The term dielectric heating where used in this specification and claimsincludes any form of heating in which heat is generated by placing adielectric material in an alternating electric field.

According to the present invention there is provided a high-frequencydielectric heating process which includes the steps of supplying highfrequency power from a source to two electrodes so that a high frequencyelectric field is set up between the electrodes, and passing adielectric material in a longitudinal direction between the electrodes,the arrangement being such that one of the electrodes is effectivelydivided electrically with respect to the high frequency power into aplurality of parts arranged across the width of the material so thateach part covers a portion of the material not covered by another partduring the passage of the material between the electrodes whereby powerfrom the source is developed as heat in the material across differentportions of the width of the material each corresponding to a respectiveone of the said parts.

According to the present invention there is further provided highfrequency dielectric heating apparatus including a first electrode, asecond electrode arranged to hold a dielectric sheet material againstthe first electrode, means to cause relative movement between thematerial and the electrodes, a source of high frequency electricalpower, and electrical connections between the source and the electrodes,the arrangement being such that in operation one of the electrodes iseffectively divided electrically with respect to high frequency powerfrom the source into a plurality of parts arranged across the width ofthe material so that each part covers a portion of the material notcovered by another part, whereby power (from the source is developed asheat in the material across different portions of the width of thematerial each corresponding to a respective one of the said parts.

Embodiments of the invention will now be described with reference to theaccompanying drawings which show,

In FIGURE 1 a plan view of an arrangment of roller electrodes,

In FIGURE 2 a diagrammatic end view of the roller electrodes shown inFIGURE 1,

In FIGURES 3, 4, 5 and 7 diagrammatic side elevations of rollerelectrode arrangements together with their circuit connections, and

In FIGURE 6 a diagrammatic plan view of a roller electrode arrangementtogether with its circuit connections.

Referring to FIGURES 1 and 2 there is shown a first electrode whichconsists of a metal roller 1 having a raised pattern 2 on its surface.Arranged on an are about the roller 1 there is a second electrodecomposed of three metal pressure rollers 3, 4 and 5. These pressurerollers are carried on shafts 6, 7 and 8 of insulating material. Theshafts are of smaller diameter than the pressure rollers so that when asheet of insulating material is passed between the roller 1 and therollers 3, 4 and 5 it is pressed against the pattern 2 over only a thirdof its width by each pressure roller. In operation the roller 1 whichforms one electrode of a dielectric heating apparatus is connected toone terminal of a source of high frequency heating power. Anotherterminal of the source is connected to one of the pressure rollers, forexample to roller 3. The connections are made from the source by thenormal type of roller contact, and are not illustrated in the figures. Asecond source is connected between the roller 1 and the roller 4, and athird source is connected between the roller 1 and the roller 5. Thesources may all be completely independent generators or they may beseparate secondary windings on the output of a single generator.

In operation thermoplastic dielectric material is fed between therollers 3, 4, 5, and roller 2 which is driven. Power from the sources ata frequency which matches the load provided by the dielectric materialis fed to the electrodes and the dielectric material is heated to such atemperature that the pattern which is imprinted or embossed into it bythe roller 1 remains in the material upon cooling.

From the above description it can be seen that the second electrode isdivided electrically with respect to the high frequency power into aplurality of parts formed by the rollers 3, 4, and 5 so that heat isdeveloped in the dielectric material across portions of the width of thematerial corresponding to respective ones of the parts of the secondelectrode.

The electrodes need not be rollers they could be flat platesand if thepattern to be imprinted were to consist of straight lines running alongthe length of the material the raised portions could be provided forexample by wheels or rollers, and the material could be fed by someother means.

It could of course be arranged that the large roller forming the firstelectrode was smooth and that each of the parts of the second electrodehad a raised pattern on its surface and that these parts were gearedtogether.

The thermoplastic dielectric material could be a single sheet of aparticular material or a laminate consisting of a foamed plastics layerbetween two sheets of plastics material. In the latter case the patternand the spacings of the rollers could be such that the two outer sheetsand the foamed plastics layer were welded together in accordance withthe raised pattern on the surface of the roller 1.

It is possible by making the length of each pressure roller between 13to 20 inches and by using a separate one kilowatt generator atfrequencies between 50 and 150 megacycles for each source to matchadequately each of the loads provided between the pressure rollers andthe roller 1 by plastics material such as polyesters, polyamides oracetates and to weld material of a total width of 40 to 60 inches.

The ability to use higher frequencies may be seen from the fact that fora weld of a particular width, for example one eighth of an inch wide, itis possible to use a frequency of 70 mc./s. over a length of twentyinches, however,'for a length of forty inches of the same width offrequency of 36 mc./s. may be necessary, and for a length of fiftyinches it is necessary to use a frequency of 20 mc./s. The disadvantageof using a lower frequency is that it is necessary to increase the powerto be supplied to the materialto obtain the same degree of heating. Thevoltage between the welding electrodes is also increased and thepossibility, especially with thin materials, of a breakdown through thematerial is therefore increased.

The pressure rollers may of course be sleeves arranged on theirrespective shafts, and their positions on the shafts may-be madeadjustable.

In order to avoid interference it may be arranged that each of thesources is at a different frequency.

Referring to FIGURE 3 there is shown an arrangement including a firstelectrode consisting of a roller 10 having a raised pattern on itssurface and two pressure rollers 11 and 12 forming a second electrode.The rollers 11 and 12 are displaced around the periphery of the roller10 but effectively cover a length of the roller 10 equal to the -widthof a plastics material to be embossed. Sources 13 and 14 of highfrequency electrical power are connected respectively between rollers 11and 1t and 12 and 10. The operation of this arrangement is similar tothat described with reference to FIGURES l and 2. When plastics sheetmaterial is passed between roller 10 and the pressure rollers as theresult of a drive being applied to the roller 10 the material isembossed or in the case of a laminate material welded according to theraised pattern in two parts each extending across a part of the widthand corresponding to the two parts of the second electrode.

With reference to FIGURE 4 an arrangement is shown in which there is afirst electrode formed by roller 15 and a second electrode formed bypressure roller 16 connected at different points to sources of highfrequency electrical power 17 and 18. The other terminals of the sourcesare connected to roller 15. The connections are such that as a result ofthe loads presented the roller 16 is effectively divided into two partswith respect to each of the sources. Thus power from one of the sourcesis dissipated in the plastics material fed between the rollers at pointscorresponding to one of the parts while power from the other of thesources is caused to heat the material at points which correspond to theother of the parts.

It could be arranged for a single source to be used and for feed pointsfrom the source to be connected at intervals along the length of theroller 16 so that the roller was effectively divided electrically withrespect to the high frequency source into a plurality of parts. Each ofsuch parts is capable of being matched to a source of much higherfrequency than it would be possible to use were the electrode to appearto the source to be a single roller, equal in length to the combinedlengths of the parts and each of the parts causes heat to be developedin the material according to the electric field associated with itacross a portion of the width of the material corresponding to therespective part.

In FIGURE an arrangement of rollersZt), 21 and 22 is shown which issimilar to that described with reference to FIGURE 3. However, theconnections from the source 23 are arranged so that power from thesource is divided between the load provided by the plastics materialpassed between the rollers in two parts. Thus the power dissipatedbeneath electrode 21 heats one part of the width of the material and thepower dissipated beneath electrode 22 heats a further portion of thewidth of the material.

Referring to FIGURE 6 there are shown four composite rollers havingelectrode portions 25, 26, 27 and 28 and portions 29, 30, 31 and 32 madeof insulating material. The rollers are arranged in line and electrodeportions 25 and 27 have raised patterns on their surfaces.

A source 33 of high frequency electrical power is connected to electrodeportions 25 and 28 and in operation the rollers are geared together sothat a sheet of plastics material may be fed in one direction betweenelectrodes 25 and 26 in the opposite direction between electrodes 26 and27 and back again in the original direction, between electrodes 27 and28. The electrodes form effectively the plats of a number of capacitorsin series and the plastics material is the dielectric of the capacitor.It can be seen that both electrodes 26 and 27 are effectively dividedinto two parts. One of the parts of electrode 26 co-operates Withelectrode 25 and the other part with electrode 27. The other part ofelectrode 27 co-operates with electrode 28. Thus both electrodes 26 and27 are divided into two parts so that power is developed as heat in theplastics material across a portion only of the width of the materialcorresponding to a respective one of the parts.

With reference to FIGURE 7 it can be seen that a first electrode formedby rollers 35 and 36 is divided into two parts. The rollers 35 and 36are geared together and have a raised pattern on their surfaces.Pressure roller 37 forms the other electrode of the arrangement. Highfrequency electrical power from a source 39 is split, by the centre tapconnection 40 made between the source and roller 32 and the endconnections 41 and 42 made between the source and rollers 35 and 36,into two parts. The power developed in any plastics dielectric materialpassed between the electrodes is thus divided into two parts eachextending across a portion only of the width of the material andcorresponding to the division into two parts of the first electrode.

It has been found that use of the invention provides a better control ofthe heat generated across the width of a sheet of material than waspossible previously, and that by the use of higher frequenciesdifiiculties caused by standing waves or other reasons are reduced.

Although the invention has been described in relation i to embossing andwelding its use is not limited to these applications and it may be usedfor other purposes such as drying materials during a step in production.

It is of course necessary for maximum efiiciency for the loads presentedby each of the parts of the electrode and its respective portion ofmaterial to be matched acourately to the source of power at theappropriate frequency so that the degree of heating may be properlycontrolled.

I claim:

1. A high-frequency dielectric heating process which includes the stepsof supplying high frequency power from a source to two electrodes sothat a high frequency electric field is set up between the electrodes,and passing a dielectric material in a longitudinal directi on'betweenthe electrodes, the arrangement being such that one of the electrodes iseffectively divided electrically with respect to the high frequencypower into a plurality of parts arranged across the width of thematerial so that each part covers a, portion of the material not coveredby another 3 part during the passage of the material between the.electrodes whereby power. from the source is developed as heat in thematerial across different portions of the width of the material eachcorresponding to a respective one of the said parts.

2. A process as claimed in parts of the electrodes are one another.

3. A process as claimed in claim 1 in which the said parts of theelectrode are connected together electrically with respect to a directcurrent.

4. High frequency dielectric heating apparatus including a firstelectrode, a second electrode arranged to press a dielectric sheetmaterial towards the first electrode, means to cause relative movementbetween the material and the electrodes, a source of high frequencyelectrical power, and electrical connections between the source and theelectrodes, the arrangement being such that in operation one of theelectrodes is effectively divided electrically with respect to highfrequency power from the source into a plurality of parts arrangedacross the width of the material so that each part covers a portion ofthe material not covered by another part, whereby power from the sourceis developed as heat in the material across different portions of thewidth of the material each corresponding to a respective one of the saidparts.

5. High frequency dielectric heating apparatus as claimed in claim 4wherein one of the electrodes has portions of its surface raised.

6. High frequency dielectric heating apparatus claim 1 in which the saidelectrically insulated from References Cited UNITED STATES PATENTS2,525,355 10/1950 Hoyler 219-1081 X 2,542,702 2/1951 Prow 219-10.81 X2,616,015 10/1952 Kinn 219-1081 2,766,362 10/ 1956 Kinder et al219-10.61 X

FOREIGN PATENTS 493,750 6/1953 Canada. 484,804 12/ 1953 Italy.

RICHARD M. WOOD, Primary Examiner. ANTHONY BARTIS, Examiner. L. H.BENDER, Assistant Examiner.

1. A HIGH-FREQUENCY DIELECTRIC HEATING PROCESS WHICH INCLUDES THE STEPSOF SUPPLYING HIGH FREQUENCY POWER FROM A SOURCE TO TWO ELECTRODE SO THATA HIGH FREQUENCY ELECTRIC FIELD IS SET UP BETWEEN THE ELECTRODES, ANDPASSING A DIELECTRIC MATERIAL IN A LONGITUDINAL DIRECTION BETWEEN THEELECTRODES, THE ARRANGEMENT BEING SUCH THAT ONE OF THE ELECTRODES ISEFFECTIVELY DIVIDED ELECTRICALLY WITH RESPECT TO THE HIGH FREQUENCYPOWER INTO A PLURALITY OF PARTS ARRANGED ACROSS THE WIDTH OF THEMATERIAL SO THAT EACH PART COVERS A PORTION OF THE MATERIAL NOT COVEREDBY ANOTHER PART DURING THE PASSAGE OF THE MATERIAL BETWEEN THEELECTRODES WHEREBY POWER FROM THE SOURCE IS DEVELOPED AS HEAT IN THEMATERIAL ACROSS DIFFERENT PORTIONS OF THE WIDTH OF THE MATERIAL EACHCORRESPONDING TO A RESPECTIVE ONE OF THE SAID PARTS.